Cellular immunity to pseudomonas antigens is evaluated by in vitro lymphocyte proliferation assays using antibiotic killed Pseudomonas aeruginosa as the antigenic stimulus. Certain patients with Cystic Fibrosis have abnormally low or absent lymphocyte responses to pseudomonas antigen. Low or absent responses to pseudomonas antigen is found to correlate with progression of pulmonary disease and poor long term prognosis. The mechanism of the development of lymphocyte unresponsiveness and means of reversing unresponsiveness to pseudomonas are being investigated.